The subject invention relates to a method and system for the printing of documents which are difficult to reproduce without detection. More particularly, it relates to printing of documents which have value using a thermal computer printer.
Documents which represent proof of payment (e.g., mail pieces which have been franked with a postal indicium, tax stamps) or which have a monetary value (e.g., tickets) must be secure, i.e., difficult to counterfeit. Printing such documents with a conventional computer printer is difficult, because methods for precise duplication are widely available using off-the-shelf scanning and printing technologies.
One method for solving this problem is to create complex patterns or images on the document, or to embed information in an image or pattern on a document in a manner such that the images or patterns cannot be reproduced with sufficient precision using widely available technologies. These methods, however, require that very high quality printing technology be used to produce the original documents.
Another method is to use a unique stock as the substrate upon which a document is printed. The unique features of the stock help serve to authenticate the original document; the best-known example being the unique stocks used to print currency. Such unique stocks, however, are expensive and require careful management.
While methods such as these have proven useful, their expense become hard to justify when the inherent value or the number of documents is small. For example, postage metering systems, which are well known systems for franking mail pieces as proof of payment of postage, are frequently used by small mailers who mail from a few to a few tens of mail pieces a day, which will not readily support the expense of high quality printing technologies or unique, controlled stock. Thus, it is desirable to use substantially conventional computer printing technologies for postage meters and similar applications. Yet at the same time cumulative postage, particularly for high volume mailers who may mail thousands of mail pieces daily, provides a significant inducement to fraud. Furthermore, while small mailers do not often mail a large number of mail pieces, in certain cases the total postage associated with those mail pieces may be significant enough (e.g., if all the pieces require special handling, e.g. Express Mail®) to induce the small mailer to commit fraud.
FIG. 1 shows a simplified representation of conventional postage meter 10; including a secure accounting system 12, thermal printer controller 14 and thermal print head 16. (Print head 16 is a contact thermal print head where its surface is in sliding contact with the surface of print stock 20 during the printing process; and as used hereinafter the terms “thermal print head” or “thermal printer” refer to such contact printing systems.) Postal information, including a postage amount that will be incorporated into a postal indicium (not shown), is input to accounting system 12. System 12 records the postage amount to assure payment of postage costs. Typically the postage amount is deducted from a pre-paid sum with which meter 10 is “filled”, but in other meters the postage amount is simply recorded for later payment. After the postage amount is recorded, the amount, and possibly other information to be incorporated into a postal indicium, are input to printer controller 14, which formats the indicium, typically including fixed graphical elements, the postage amount, and information such as the date, meter serial number, etc. Controller 14 then controls print head 16 to print the indicium on thermal print stock 20.
Print stock 20 (shown in cross-section) includes a polymer top coating 22, thermal ink layer 24 and substrate 28. In operation conventional mechanisms (not shown) move print head 16 and stock 20 so that print head 16 moves through a predetermined raster pattern with respect to stock 20.
FIG. 2 shows a more detailed illustration of the operation of print head 16. Print head 16 (shown partly broken away in FIG. 2) comprises heater elements 16H-1 through 16H-n. Selected ones of heater elements 16H (here elements 16H-1 and 16H-(n−1)) are energized and heated to a threshold temperature for selected time periods by controller 14 as head 16 moves through the predetermined raster pattern to develop regions PE of ink layer 24; thus forming corresponding printed elements of the postal indicium image. Conventionally selected ones of elements 16H are energized by strobing a pulse of predetermined voltage and duration from a power supply output to a selected element, synchronously with the movement of head 16, according to whether or not a corresponding element of a bitmap representative of the indicium is asserted. Note that top coating 22, which is manufactured with a high degree of polish, is substantially unaffected by the normal printing process. Other aspects of postage meter operation, such as security features to prevent tampering with accounting system 12 and the use of encryption technology to uniquely identify postal indicia, are well known to those skilled in the art and need not be discussed further here for an understanding of the subject invention.
Many recently developed postage meter systems are based on conventional microprocessor based computers (e.g., “PCs”) and use conventional computer printers to print postal indicia. Thus, controller 14 and print head 16 typically will comprise, or be substantially similar to, an off-the-shelf computer thermal printer. Thus, as discussed above, readily available scanning technology and off-the-shelf printers easily can be used to produce fraudulent duplicate postal indicia, or other documents of value whose production is controlled. While in principle it is possible to detect the use of duplicates of unique documents such as postal indicia, in practice this has proven difficult.
Thus it is an object of the subject invention to provide a method and system for printing secure documents using substantially off-the-shelf computer printing technology.